Lunar and Planetary Science XXXIX (2008)
1806.pdf
PHYLLOSILICATE AND HYDRATED SULFATE DEPOSITS IN MERIDIANI. S. M. Wiseman1, R. E.
Avidson1, S. Murchie2, F. Poulet3, J. C. Andrews-Hanna4, R. V. Morris5, F. P. Seelos2, and the CRISM Team. 1Dept
of Earth and Planetary Sciences, Washington University, St. Louis, MO (sandraw@levee.wustl.edu), 2Applied
Physics Laboratory, Laurel, MD, 3Institut d’ Astrophysique Spatiale (IAS), Orsay, France, 4Dept of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA, 5NASA Johnson Space Center, Houston, TX.
Introduction: Several phyllosilicate and hydrated
sulfate deposits in Meridiani have been mapped in
detail with high resolution MRO CRISM [1] data. Previous studies have documented extensive exposures of
outcrop in Meridiani (fig 1), or ‘etched terrain’ (ET),
that has been interpreted to be sedimentary in origin
[e.g., 2,3]. These deposits have been mapped at a regional scale with OMEGA data and show enhanced
hydration (1.9µm absorption) in several areas [4].
However, hydrated sulfate detections were restricted to
valley exposures in northern Meridiani ET [5]. New
high resolution CRISM images show that hydrated
sulfates are present in several spatially isolated exposures throughout the ET (fig 1). The hydrated sulfate
deposits in the valley are vertically heterogeneous with
layers of mono and polyhydrated sulfates and are morphologically distinct from other areas of the ET. We
are currently mapping the detailed spatial distribution
of sulfates and searching for distinct geochemical horizons that may be traced back to differential ground
water recharge and/or evaporative loss rates.
The high resolution CRISM data has allowed us to
map out several phyllosilicate deposits within the fluvially dissected Noachian cratered terrain (DCT) to the
south and west of the hematite-bearing plains (Ph) and
ET (fig 1). In Miyamoto crater, phyllosilicates are located within ~30km of the edge of Ph, which is presumably underlain by acid sulfate deposits similar to
those explored by the Opportunity rover. The deposits
within this crater may record the transition from fluvial
conditions which produced and/or preserved phyllosilicates deposits to a progressively acid sulfate dominated groundwater system in which large accumulations of sulfate-rich evaporites were deposited .
crater in northern Meridiani as well as in a portion of
the ET to the east [4] (fig 1). However, the geologic
context of these deposits was difficult to decipher.
Using CRISM and other orbital data, we have defined
a lateral transition from phyllosilicate bearing material
to layered terrain that occurs within the heavily modified ~130km diameter Miyamoto crater. Phyllosilicates (fig 2a) are present within crater deposits ~30km
from the edge of the layered terrain explored by the
Opportunity rover at the landing site ~150km to the
northeast (fig 1). Phyllosilicate exposures exhibit spectral features resulting from Fe/Mg-OH vibrations at
2.3µm and bound H2O at 1.9µm. The band center and
shape of the ~2.3µm absorption feature indicates the
presence of saponite, a smectite clay (fig 2d).
A geologic cross section of this area shows that
Miyamoto crater was heavily infilled prior to the emplacement of the layered deposits (fig 2e). Analysis of
HRSC data show that phyllosilicate exposures occur in
areas where a capping unit has been stripped away (fig
2b). This capping unit appears to unconformably overlie the phyllosilicate bearing material and extends to
the edge of and underlies the layered terrain to the
east. This relationship indicates that the phyllosilicates
predate the emplacement of the layered terrain. The
phyllosilicate deposits within the crater in northern
Meridiani, which are spectrally similar to those within
Miyamoto, may also predate the emplacement of ET.
a)
HRSC H2097
b)
Ph
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Figure 1. Regional map of Meridiani. Phyllosilicate detec-
tions are shown in green and hydrated sulfates in red.
Phyllosilicates: Phyllosilicates have been previously detected using OMEGA data within a large
2 km
R=2.3, G=1.5,
B=1.1 µm
Reflectance
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ET
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Opportunity
HRSC H2097
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Chlorite
Saponite
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Wavelength (microns)
Lunar and Planetary Science XXXIX (2008)
Elevation (m)
e)
1806.pdf
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Phyllosilicatebearing Material
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Hematitebearing plains
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Figure 2. a) THEMIS DIR mosaic of Miyamoto crater with
CRISM phyllosilicate detections shown in green, b) HRSC
2097_0000 subset showing capping unit (orange), phyllosilicates (green), and hematite-bearing plains (blue), c) CRISM
FRT 7B8B falsecolor image, d) CRISM ratio spectrum and
library spectra, e) MOLA cross section.
Hydrologic modeling [6] suggests that layered deposits would have formed throughout the crater interior and only later been eroded back to the present
extent to expose the pre-existing phyllosilicate-bearing
material. The spatial juxtaposition of phyllosilicate and
acid sulfate deposits provides a potentially important
site for exploration by upcoming rover based missions,
and is currently a candidate MSL landing site.
Hydrated Sulfates: Mono and polyhydrated sulfates (with absorption features at 2.1 + 2.4µm and 1.9
+ 2.4µm, respectively) were originally mapped within
and near the northern Meridiani valley with OMEGA
data [7]. Analyses of high resolution CRISM, HiRISE,
and CTX images show that several spectrally and
geomorphically distinct layers occur within the valley
at the 10s of meters scale that have been exposed by
differential erosion [8]. The apparent active erosion in
this area may be responsible for exposing relatively
fresh hydrated minerals.
Hydrated sulfate deposits have also been detected
with CRISM at small spatial scales in several locations
throughout the ET (fig 1). We are currently mapping
these marker minerals. Initial results suggest that
some of these small exposures represent remains of
more coherent units that have been stripped away or
obscured from detection by overlying material. Deposits similar to the material exposed in the valley may
occur at the base of the stratigraphic section explored
by the Opportunity rover.
Opportunity rover observations show that hematitic
concretions weathered from outcrop (ET) that consists
of layers of dirty evaporites that formed in an acid sulfate environment [9,10,11]. The hydroxylated sulfate
mineral jarosite occurs within the outcrop [12]. Orbital
CRISM spectra of the outcrop exposed along the rover
traverse are dominated by nanophase ferric oxide features and do not show distinct features related to hydroxylated or hydrated sulfates [14].
The lack of sulfate detection in CRISM data may
be explained by the presence of coatings thick enough
to obscure reflectance signatures of underlying sulfates, but thin enough to be transparent to the gama
radiation used by the MB, and/or the fine grain size of
the sulfates precludes detection using VIS/SWIR data.
Discussion: The deposits in Meridiani record the
waning of the hydrologic cycle on Mars. High energy
erosive conditions which degraded and dissected the
Noachian cratered terrain transitioned to a ground water dominated regime in which evaporation and aeolian
reworking dominated to produce thick accumulations
of layered outcrop (ET). Phyllosilicate deposits appear
to be associated with the most ancient rocks on Mars
[15] and hydrated sulfate deposits are present in some
of the late Noachian or early Hesperian layered terrain
in Valles Mariners and Meridiani. The transition in
hydrologic regimes on Mars to an acid sulfate ground
water system may be recorded in the phyllosilicate and
layered deposits within Miyamoto crater.
Several distinct mineral assemblages are present
within the layered outcrop material in Meridiani that
have been grouped together as ET. Acid sulfate conditions are suggested by the presence of jarosite within
the outcrop measured by the Opportunity rover. The
hematitic spherules are derived from the outcrop, and
the presence of the laterally extensive Ph indicates that
much of the outcrop underlying this unit is similar in
nature to the deposits explored by the rover. Areas of
the ET that exhibit hydrated sulfate signatures experienced distinct geochemical conditions during deposition and/or diagenesis that preserved these minerals in
measurable quantities. Active erosion may contribute
to the orbital detection of these deposits.
The geochemical conditions experienced by ET at
various locations were driven by ground water recharge and evaporative loss rates, both of which influence the water to rock ratio and therefore the fluid
chemistry. Hydrologic modeling [6] predicts regions of
groundwater upwelling and variation in evaporative
loss rates throughout the Meridiani region. Continued
high resolution spectral and geomorphic mapping of
distinct terrains, including phyllosilicates and sulfates,
within Meridiani will lead to a better understanding of
the local and global scale hydrologic regime on Mars.
References: [1] S. Murchie et al. (2007) JGR, 112.
[2] M. Malin and K. Edgett, (2000) Science, 288. [3]
P.R. Christensen and S.W. Ruff (2004) JGR, 107. [4]
F. Poulet et al. (2007) Icarus, in press. [5] A. Gendrin
et al. (2005) Science, 307. [6] J.C. Andrews-Hanna
and M.T. Zuber (2008), this conf. [7] J.L. Griffes et al.
(2007) JGR, 112. [8] S.M. Wiseman et al. 7th Mars
abst#3111. [9] S.W. Squyres et al. (2004) Science,
306. [10] R.E. Arvidson et al. (2006) JGR, 111. [11] J.
Grotzinger et al. (2005) E.P.S.L., 240.[12] R.V. Morris
et al. (2003) Science, 306. [13] T. Glotch et al., (2006)
JGR, 111. [14] S. Murchie. et al. (2007) Nature,
submitted. [15] J-P. Bibring et al. (2006) Science, 312.